Uranium and Thorium Distributions in the Sediments Collected from the Western South

1. Fig. 2. U-238, U-234 and U-234/U-238 activity ratio determined from cores at Stations C, D and E. Fig. 5. U-238, U-234 and U-234/U-238 activity ratio determined from cores at T17, T18 and T19. Fig. 3. Th-232, Th-230 and Th-228 determined from cores at C, D and E. Fig. 6. Th-232, Th-230 and Th-228 determined from cores at T17, T18 and T19. Fig. 7. Th-230/Th-232 and Th-228/Th-232 activity ratio determined from cores at T17, T18 and T19. Fig. 4. Th-230/Th-232 and Th-228/Th-232 activity ratio determined from cores at C, D and E. OS41A-04 Uranium and Thorium Distributions in the Sediments Collected from the Western South Okinawa Trough and the Northern South China Sea Chun-Yen Wang, Li-Wen Wang and Yu-Chia Chung Institute of Marine Geology and Chemistry, National Sun Yat-sen University, Kaohsiung, Taiwan Abstract Uranium and thorium radionuclides were measured on three cores each collected from the western South Okinawa Trough (SOT) and northern South China Sea (SCS) in order to examine the variations of these radionuclides and their activity ratios in the sediments and to understand the geochemistry of these nuclides in these two marginal sea areas. Three box cores were taken from the basin of the northern SCS, and three gravity cores were collected from western SOT. The U-238 and U-234 activities in these cores show no significant vertical nor regional variations with activities ranging between 1.3 and 2 dpm/g and the U-234/U-238 activity ratio of about 1.1, quite close to the value of seawater (1.14). However, the activities of thorium isotopes are quite different between these two regions. In the northern SCS, Th-232 has a mean activity of about 2.5 dpm/g, and Th-228, the granddaughter of Th-232, has a similar mean value. Th-230 has activities generally ranging from 2 to 6 dpm/g, showing an excess over U-234. The Th-230/Th-232 activity ratio falls generally between 1 and 3. Th-232 and Th-228 in the western SOT show higher activities (~3.5 dpm/g) than those (~2.5 dpm/g) in the northern SCS. Th-230 in the western SOT is about 2 to 2.5 dpm/g, resulting in a lower Th-230/Th-232 activity ratio (～0.6-0.7) in this region. Since U-238 and U-234 are quite comparable between the two regions, the higher activity of Th-230 in excess over U-234 in the northern SCS may be due to greater water depth that allows more Th-230 to be scavenged from the water column. Introduction Uranium and thorium isotopes have been used commonly as radiotracers for studies of marine sediments, including their sources, transport and sedimentation rates in relation to the sedimentary and geochemical processes in a marine environment. During settling, U-234 in the particulates or sediments may be leached out preferentially over U-238 due to recoil, causing an excess in U-234 over U-238 activity in the ocean water and sometimes U-234 deficit in the sediments. Thorium isotopes include Th-232 of terrigenous or dertital origin, Th-230 decayed from U-234, and Th-228 produced from Ra-228 decay. The purpose of this study is to present some U and Th isotope data measured from the sediments collected from two major marginal seas, the northern South China Sea (SCS) and the western South Okinawa Trough (SOT), and to understand their geochemical implications. The SCS is a semi-enclosed marginal sea west of the tropical western Pacific, with its most terrigenous sediments contributed by major rivers such as the Pearl and Mekong rivers. A basin over 4000 m deep is located in the center, and large shelf areas occupy the northern and southern margins. The cores were taken in the central basin (Fig. 1) where sedimentation rates are known to be low. The coring sites in the western SOT are located just off northeastern Taiwan. The hydrography and the current system in both regions have been studied quite extensively over the years. Sampling and analytical methods Six cores taken from the study area (Fig. 1) were analyzed for their uranium and thorium isotopes. The box cores from SCS were subcored on board the R/V Ocean Researcher I and returned to the shore-based laboratory. The gravity cores from Stations T17 and T19 were sampled at 1 cm interval while that from Station T18 at 2 cm interval. The subcores taken from Stations C, D and E were sampled at 1 cm interval in the upper 10 cm or so and then at 2 cm interval or more below. The water content of each sample in percent was determined gravimetrically. The desalted sample was dried and ground to homogeneous powder for later analyses. The radiochemical procedures for measurements of these isotopes follow mainly that of Anderson (1981). Briefly, a sample of about 2 g taken from the powdered sediments was spiked with known U-236 and Th-229 activities, respectively, as U and Th yield tracers. The sample was treated with a mixture of HCl, HClO4, and HF to achieve total dissolution. Care was taken to remove any residual HF which could form stable, undissolvable fluorides before amonium hydroxide was added (to pH~8). Uranium and thorium were coprecipitated with Fe(OH)3 and Al(OH)3. The precipitates were then dissolved in 9N HCl, and separation and purification of these two elements were carried out by standard ion exchange procedures. Finally, U and Th isotopes were electroplated separately onto stainless steel dics and their alpha activities were measured with a silicon surface barrier detector coupled with a multi-channel analyzer. Fig. 1. Locations of the three box cores (C, D, E) taken in the SCS and three gravity cores (T17, T18, T19) in the western SOT. Results and discussion ◎ South China Sea (SCS) Uranium and thorium isotopes are quite variable in this area. U-238 varies generally from about 0.5 to 1.7 dpm/g with the lowest values (～0.6 dpm/g) at station E (Fig. 2). U-234 mimicks U-238 with slightly higher activities, resulting in a mean U-234/U-238 activity ratio of about 1.1, comparable to that in seawater (1.14). Th-232 at Stations C and D varies between 2 and 3 dpm/g, but that at Station E is systematically lower (all lower than 1 dpm/g) (Fig. 3). Th-230 has highest activities at Station D and lowest at Station E. Both Stations C and D show clear Th-230 excess over U-234 due to input from the deep water column by particulate scavenging. Little Th-230 excess is found at station E. Th-228 behaves similarly to Th-232 but with slightly higher activity, resulting in a Th-228/Th232 activity ratio higher than 1 (Fig. 4). The Th-230/Th-232 activity ratio varies generally from 1 to 3 (Fig. 4). These values are much higher than those of the SOT cores as will be shown later. ◎ South Okinawa Trough U-238 and U-234 show similar vertical distribution at each station, but they are systematically lower by about 0.5 dpm/g (1.4 vs 1.9 dpm/g) at T19. The U-234/U-238 activity ratios are quite uniform at about 1 for all station (Fig. 5). The particulates collected by sediment traps in the same area yield similar values. Uranium contents in terrigenous materials from Taiwan are about 1.9 dpm/g, same as those observed at T17 and T18. Narita et al. (1990) reported U-238 values at about 1.44 dpm/g in the Okinawa Trough. Our T19 values (～1.4 dpm/g) are comparable to those reported by Narita et al. (1990). Th-232 in marine sediments owes its source or origin solely from the continental landmass. The downcore thorium isotope variation at each station is quite small (Fig. 6). Th-232 tends to decrease seaward from T17 to T19 (3.9 to 3.5 dpm/g). There is little excess Th-230 (～0.5 dpm/g) over its parent, U-234. The Th-230/Th-232 activity ratios are fairly uniform for each station (about 0.7 at T17, T18 and 0.55 at T19, see Fig. 7). The Th-228/Th-232 activity ratios are fairly uniform at about 0.9, suggesting a small Ra-228 loss in the sediments. Fig. 6. Th-232, Th-230 and Th-228 determined from cores at T17, T18 and T19. Fig. 5. U-238, U-234 and U-234/U-238 activity ratio determined from cores at T17, T18 and T19. Conclusions Uranium in the sediments of the SOT area is higher than that of the SCS area by about 0.5 to 0.7 dpm/g. The U-234/U-238 activity ratios tend to be slightly greater than unity with a mean of 1.1, suggesting that additional uranium may have precipitated from the pore water in which U-234 is enriched. Th-232 in the SOT cores shows no vertical variation, with activity falling between 3.5and 3.9 dpm/g. The SCS sediments have lower Th-232 (at 2 to 3 dpm/g), but values lower than 1 dpm/g are observed in the entire core at Station E. Both Th-230 and Th-228 are also the lowest at Station E. These suggest that the sediments in the two marginal seas have different sources but the near constancy of Th-232 at each region implies a relatively uniform depositional environment within the time span of the cores. Th-230 is higher and more variable in the SCS than in the SOT cores due to greater water depth and smaller sedimentation rate in the SCS . The Th-228/Th-232 activity ratio in the SCS sediments is greater than unity, suggesting additional contribution from Ra-228 decay in the pore water. The Th-230/Th-232 activity ratio ranges from 0.55 to 0.7 in the SOT cores but it varies generally between 1 and 3 in the SCS sediments. In the SCS deep basin over 4000 m, the Th-230 produced from U-234 decay can be scavenged effectively from the water column and deposited as an excess over U-234 in the sediments. On the contrary, very little excess Th-230 is observed in the SOT cores.